Electrostatic filter and supply air terminal

ABSTRACT

An electrostatic filter for the separation of particles includes a particle charging unit arranged upstream, including at least one corona wire for the charging of particles and a particle collector unit arranged downstream relative to it. The particles collecting unit includes two groups of plate-like elements (1, 1&#39;) arranged essentially parallel between each other and the flow direction through the device at a predetermined distance from each other, wherein a first group of elements (1)is connected to earth (3) and a second group elements (1&#39;) is electrically insulated with reference to the device and to the first group of elements (1) and that the elements (1, 1&#39;) essentially are made of one or more non-hygroscopic polymer materials with a mass resistivity which preferably lies within the region of 1×10 2  Ωcm to 1×10 8  Ωcm. In one embodiment the elements are alternatively offset to each other in the longitudinal direction of the unit, whereby the first group of earthed elements (1) are arranged further away from the particle charging part and the other group of elements (1&#39;) are electrically insulated with reference to the device and to the first group of elements (1) and arranged nearer the particle charging part.

This application is a National Stage Application of InternationalApplication No. PCT/SE96/00151 filed on Feb. 8, 1996.

FIELD OF THE INVENTION

The present invention relates to an electrostatic filter for theseparation of particles out of a flowing gaseous medium and a supply airterminal comprising such a filter. The filter comprises a particlecharging unit arranged upstream comprising at least one corona wire forthe charging of the particles and a particle collector unit arrangeddownstream relative to this. Such filters are known in the prior art.

BACKGROUND OF THE INVENTION

Normal air cleaners comprise a filter with an ionizing sector in whichthe particles comprised in the air flowing through the air cleaner areionized. i.e. receive an electrical charge. The particles then pass aparticle collector unit where the charged particles interact with anelectrical field produced by plates charged with an opposite electricalcharge. In this manner the particles are driven towards the plates andcollide with the plates and are collected.

Swedish Patent DE-B-401 327 relates to an electrostatic filter whichcomprises wireshaped conductors, so called corona wires, and plates withdifferent electrical potentials. There are particles in the mediumflowing through the filter which are charged and attracted and struckonto the plates which have the reverse polarity relative to theparticles charged during the passage between the corona wires. Aninsulating plate is arranged between each plate in order to preventspark-over between the electrically charged plates, caused amongstothers by the build-up of a layer of the particles attracted to theplates. The biggest disadvantage with such a filter is that one must usemany plates, both charged and insulating, in order to achieve a suitabletotal filter area.

Another significant disadvantage is that after a relatively short periodof time the filter loses a considerable amount of efficiency andparticle collecting capacity. By reason of this the filter must becleaned at relatively short time intervals which often is a laboriousand time-consuming job.

Another disadvantage which appears with the above type of filter withelectrically charged plates is that when the particles move in the fieldbetween the plates a number of the particles already become dischargedat a distance from the plates through spark-over and the particles thenmay possibly not stick onto the plates but instead follow the air flowthrough the filter arrangement and out into the surroundings again.

Another type of filter device is shown in SE,A,7114330-9 (372 180). Inthis a device for the separation of particles out of a gas flow isdescribed in which a number of high voltage electrodes and collectorelements for the charged particles are alternatively arranged one afterthe other in the direction of the gas flow. The collecting elementsconsist of a network of paper, possibly metallized, which is earthed.The paper in the network has a weak conductivity.

It has now become apparent that the paper used for this purpose has alarge disadvantage in that it is hygroscopic because in the case thatthe paper absorbs moisture it changes its characteristics with respectto its conductivity.

From JP-61-164664(A) a filter is known comprising corona electrodes andopposing electrodes which are electrically conducting and connected tothe opposing pole (earth) of a voltage source with respect to theconnection of the corona wires. Between these "opposingly charged"electrodes "acceleration electrodes" made of electrically insulatingmaterial are arranged. The distance between the corona electrode and the"opposingly charged" (earthed) electrodes is shorter than thecorresponding distance between the corona electrode and the accelerationelectrodes made of the electrically insulating material.

In this field it is also known to use sheet aluminum in the particlecollection plates. This has, however, the disadvantage that during useit oxidizes. This means that the surface of the plates becomes more andmore electrically insulating with a reduced particle collecting effectas a result. During the cleaning/regenerating of these plates stronglybasic cleaning solutions must be used in order to destroy the aluminumoxide. This is obviously a big disadvantage.

Amongst the advantages of electrostatic filters is their relatively goodparticle collection capacity, at least when they are new. The abovedescribed filters, however, show a plurality of disadvantages, amongstothers the difficulty of cleaning the particle collecting part forreuse. These difficulties are attributable to the construction of saidfilters.

In this connection it should be mentioned that electrostatic filters ofwhich the plates are made of metal or in which metal is comprised,require a coupling resistance between the plates and earth. This hasshown to be a disadvantage on one hand from the function point ofview--if one or more of the plates is short-circuited, then a large partof the filter's capacity is lost. Because of this the filter is usuallydivided into sections with respect to the plates. Furthermore thisconstruction in itself is a disadvantage from a recycling point of viewbecause a number of parts constructed of different materials must betaken apart and sorted, e.g. metal plates, resistance wires, condensorsand plastic rails in addition to the casing and other associatedcomponents.

Further, in the applicant's own application SE,A,9303059 with the filingdate Sep. 13, 1993 an electrostatic filter for the separation ofparticles is described which comprises a particle charging unit arrangedupstream comprising on the one hand a corona wire for the charging ofthe particles, and on the other hand a particle collector unit arranged20 downstream relative to this and which is at least partially earthed.The particle collector unit comprises a plurality of plates arranged ina casing, whereat the casing and the plates are arranged to haveessentially the same electric potential in relation to the corona wire.This is achieved through the plates and the casing being earthed and bythe material in the casing and the plates each comprising anon-hygroscopic material with high resistivity, correspondingessentially to the mass resistivity of a so called soft-earthedmaterial.

Even if the last filter solves some of the above mentioned problems, ithas surprisingly been shown that further improvements are achievablewith a filter according to the invention of the type given below.

OBJECTS OF THE INVENTION

The purpose of the present invention is to produce an improvedelectrostatic filter in which the above mentioned disadvantages areeliminated and in which a very high particle collecting efficiency canbe retained during a long period of use.

A further purpose is to achieve an electrostatic filter in which theparticles charged by means of corona discharge are guided towards theplates of the particle collecting unit without the use of control platesconnected to a voltage source. Control plates of the type mentioned, asis easily understood, lead to problems during cleaning of the unit.

A further purpose of the invention is to produce an electrostatic filterin which the filter is independent of electrically connected controlplates which necessitate insulating plates between them.

A further purpose is to reduce the number of components in theelectrostatic filter.

A further purpose is to achieve independence of the moisture content inthe gas-like flow streaming through the filter arrangement.

A further purpose is to be able to produce the filter arrangement in asimple and economical manner and at the same time achieve easy cleaningof it and to produce a product which can be recycled in a simple andcheap manner.

A further purpose is to improve the particle collecting capability inthe particle collector part.

A further purpose is to produce an improved supply air terminal which aswell as filtering the incoming air can also warm it.

SUMMARY OF THE INVENTION

The electrostatic filter according to the invention has a number ofadvantages. Because of its construction it is easy to clean and easy tomanufacture because the constituent elements in the particle collectingpart can all be formed in the same material if it is so desired andsubsequently they can be mounted in a simple way to form a unit throughthe plates being passed in one or more eventually pairwise arrangedcomb-like means whereafter the set of plates can further be united by aband being tensioned around it so that a packet is achieved.

Because the particles are charged by means of corona discharge in theparticle charging part and are subsequently discharged and collected inthe partly earthed particle collection part, the particle charging partis the only part of the arrangement where high voltage and/or voltage isrequired. In the particle collecting part only earthing is required.

By building up the particle collecting part in the form of a number ofelements united by means of the comb-like means and the compressingbands, this part can be mounted separately, like a packet, and placed inthe electrostatic filter, e.g. by sliding into a holder and at the sametime because of the characteristics of the elements no couplingresistance between earth or another voltage source is required. All thatis required is a simple earthing of the plates which should be earthed.

In order that the particles should stay on the plates it is necessarythat they should be discharged in close contact with the plates and notdischarged at a distance which is too far from the plates. A suitabledistance between the plates can be for example approximately 5-10 mm,preferably 5-8 mm.

The material of which the elements are manufactured has a massresistivity equivalent to a so-called soft-earth metal. By soft-earth orsemiconducting material this is meant, where appropriate, the physicaldescription of materials with a mass resistivity of approximately 10² to10¹¹ Ωcm. In the invention plastic materials with suitablecharacteristics, e.g. polypropene, polyethene, copolymers of this typeor the like are preferably used as soft-earth materials. These materialscan if it becomes necessary be doped with carbon powder or in any otherappropriate manner be equipped with a mixture of substances known to theman skilled in the art in order to achieve the desired conductibility.

The elements can advantageously be manufactured from one or morenon-hygroscopic polymer plastic materials with a mass resistivity whichpreferably lies within the range of 1×10² Ωcm to 1×10⁸ Ωcm, morepreferably 1×10³ Ωcm to 1×10⁷ Ωcm, more precisely 1×10³ Ωcm to 1×10⁴Ωcm.

It is of course possible that combinations of these materials occurequally in the form of mixtures as in the form of layered elements orelements with coatings. It is naturally also conceivable that theelements consist of several bandlike parts, which are manufactured frommaterials with the above mentioned characteristics and of materialswhich are compatible with each other.

During the choice of materials account must naturally be taken of theenvironment in which the filter arrangement is to be used, normalatmosphere, corrosive or the like. Account must also be taken of theburning characteristics and fire safety as well as of the gases possiblygiven off by the material in the case of fire. This is, however,something which is completely within the capability of the man skilledin the art to test.

A further advantage in the filter arrangement according to the inventionis that a number of components which are necessary in the electrostaticfilters according to the state of the art can be left out, such as thecoupling resistance, electrically connected guide plates, as well as thematerial of which the elements and, in the main, essentially the wholeof the particle collecting parts are constructed can be easily takencare of as if they were essentially one material which means that thefinal recycling becomes more cost-effective and easier to perform.

It has been shown to be relevant to the particles' ability to fasten onthe elements of the particle collecting part that the charged particles,when they pass through the particle collecting part, have a suitablespeed so they can get rid of their charge to these elements withoutbeing pushed away from them through recoil. A suitable flow-speed duringthe passage between the elements is of the order of 1 m/sec.

It is also important that the resistivity of the elements is neither toohigh nor too low because the particles' charge is not delivered toelements which have too high a resistivity and at too low a resistivitythe charge is delivered to the element at a distance away from it whichleads to a "discharging" of the particles and thereby that the particlesin both cases continue through the unit without fastening onto theelements.

An improvement can also be achieved by giving the elements a non-smooth,uneven surface, alternatively a roughened surface. This surface can beachieved in many ways, e.g. by extruding the elements which aresubsequently roughened or during moulding the mould can be sparked sothat during the moulding it makes an element with an uneven surface.

It has surprisingly been shown that the filter according to theinvention can be combined 20 with an addition in which warming of theair flowing through the filter can occur. In this way a supply airterminal is obtained which is effective with respect to the removal ofdirt and particles out of the air flowing through the filter which atleast under part of the year can consist of cold outside air andsimultaneously this air can be warmed so that a more comfortabletemperature is achieved and that the feeling of cold draughts is avoidedin the room into which the air flows.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below in more detail in connection to theappended drawings.

FIG. 1 shows a plurality of plate-like elements according to theinvention with the uniting comb-like means in a view seen from above.

FIG. 2 shows the comb-like uniting means from the side.

FIG. 3 shows a first embodiment of an electrostatic filter according tothe invention placed in a casing or a duct seen in the direction of flowof the flow with the upper side of the casing or duct removed.

FIG. 4 shows a section along the line IV-IV in FIG. 3, where, however,the upper side of the duct/casing is shown.

FIG. 5 shows a second embodiment of an electrostatic filter according tothe invention.

FIG. 6 shows a section through the filter in FIG. 5 along the lineVI--VI.

FIG. 7 shows a plate-like element intended for a supply air terminalwhich comprises the electrostatic filter according to the invention asan integrated unit.

FIG. 8 shows schematically a first embodiment of a supply air terminalcomprising the electrostatic filter according to the invention.

FIG. 9 shows schematically a second embodiment of a supply air terminalcomprising the electrostatic filter according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a filter arrangement with an electrostatic filter accordingto the invention. For the sake of clarity the other parts in the filterwhich are known, such as the outer casing, possible fan arrangement,possible control unit etc, are not shown. This arrangement isconsequently amongst others intended to be mounted in e.g. anelectrostatic filter with a fan arrangement which brings the air whichis to be cleaned through the filter arrangement. The filter cannaturally also be placed in a ventilation room where the fan is arrangedlike a normal extraction fan, i.e. not in connection with the filter.

The filter comprises two groups of shaped elements which aremanufactured from the earlier specified material. In this embodiment theelements 1, 1' are arranged in two groups whereby the elements in thetwo groups are alternatively arranged and groupwise offset withreference to each other. The elements 1 in the first group are joinedwith each other and to earth 3. This earthing can for example take placeby the elements mounted in the comb-like borders being slid into aholder in the filter casing in the form of a packet until the elementswhich are to be earthed come into contact with the flexible earth strapof conducting material, for example a porous electrically conductingelastomer or foamed plastic, or a non-rigid textile strip. It should beobvious to the skilled man how such an earthing can be designed in orderthat the elements to be earthed should make good contact with the earthstrip.

The elements 1, 1' are held at a predetermined distance apart by one ormore comb-like borders 2. These comb-like borders are shown seen fromthe side in FIG. 2. The filter arrangement is preferably held togetherthrough bands (not shown) tightened around the arrangement. These bandscan advantageously be tightened outside the comb-like borders. Thecomb-like borders are made of an insulating material so that the groupof elements 1' which are not earthed can continue to take up a potentialdependent on the filter arrangement geometry and the charging part. Moreon this follows below.

As the skilled man would understand, the number of elements is adimensioning question dependent on the through-flow speed, the size ofthe through-flow and other parameters which are experimentallydeterminable by the skilled man.

In FIG. 3 the particle collector part is shown placed in a casing 4 inwhich in addition to the particle collector part the particle chargingpart is also arranged, i.e. one or more corona wires 5 are arrangedbefore the arrangement in the flow direction of the air which is shownby arrows. Several electrically conducting, elongated corona wires 5 arewired up in such a way that they run through the casing 4 in itstransverse direction and at a distance from the elements. The unitingborder 2 and the earth-connections 3 between the elements 1 are alsoshown.

It should be noticed that the arrangement of one or more corona wirescan alternatively take place in a direction in the cross section of thecasing but at right angles to that shown in the drawings.

FIG. 4 shows the same arrangement in a cross section along line IV--IVin FIG. 3. Here it is shown how the corona wire 5 is fastened in thecasing 4 by means of an insulating attachment 6. The offset between thetwo groups of flat-shaped elements 1, 1' can be clearly seen. FIG. 3shows the earth-connection which connects the group of plate-likeelements I to earth and 7 refers to the band which in order to hold theplate-like elements 1, 1' together is tensioned around these and thecomb-like means 2.

FIG. 5 shows a second embodiment of a filter arrangement according tothe invention placed in a casing 4 in which in addition to the particlecollector part a particle charging part is arranged, i.e. a coronaelectrode 5 is arranged at a predetermined distance in front of theparticle collector part in the flow direction of the air which is shownby arrows. In the particle collector part there are two groups ofconcentric to each other alternatively arranged hollow cylindricalelements, on one hand earthed elements 1 and on the other hand insulatedelements 1'. Several electrodes 5 can naturally be arranged if this isappropriate without deviating from the inventive concept. The border 3earths the group of elements 1.

FIG. 6 shows the filter arrangement in FIG. 5 in cross section. Thecasing here has also been referenced by reference numeral 4 and theearthed elements with 1 and the insulated elements with 1'. In order tohold the concentric elements at the right distance apart, distanceelements (not shown) are used.

In a further embodiment of the invention according to the claims it isconceivable that the two groups of earthed respectively unearthedelements are not offset in relationship to each other, they can haveessentially the same size or alternatively have different lengths in thedirection of flow, i.e. the edges of the elements which are furthestaway in the direction of flow could extend equally far while the frontedges of the non-earthed elements could extend nearer to the particlecharging unit whereby the earthing of the elements which are to beearthed must be arranged in a somewhat different manner.

It should be noted that the comb-like borders can be placed as necessaryeither in pairs or alternatively on each side of the plate-likeelements. It is also conceivable that in certain special cases it wouldnot be necessary to have more than one comb-like border to hold togetherthe packet. In the case with concentrically arranged elements thecomb-like border can if desired be replaced by other forms ofelectrically non-conducting distance elements.

The particles which are to be separated out of the air flowing throughthe electrostatic filter are charged during passage through thehigh-voltage part, i.e. where they pass the corona wires. During use thegroup of elements which are not earthed and which extend forwardly pastthe earthed elements will have an electrical voltage distribution whichis dependent on the geometry and voltage on the corona wires. The edgesof these elements which are placed nearest to the corona wires will havean opposing charge to the corona wires, i.e. the same charge but notnecessarily of the same strength which the particles receive duringtheir passage through the particle charging part past the corona wires.This means that the particles will be led to the desired degree in thedirection towards the earthed elements whereby a discharging of theparticles towards earth occurs and whereby the particles are adsorbed onthe surface of the earthed elements.

Through the construction of the arrangement the elements with theiruniting band can easily be removed from the filter arrangement as apacket and cleaned in an elective manner, for example through simplerinsing, or the arrangement can be recycled.

The above advantages also apply to the embodiments of a supply airterminal which comprises the electrostatic element according to theinvention as an integrated unit and as shown in FIG. 7 to FIG. 9.

FIG. 7 shows a plate-like element 20, 20' intended for a supply airterminal. This element comprises three different zones of polymerplastic material with different characteristics. The first zone 10 isformed from the material which is described above in connection to theelements 1 respectively 1' comprised in the particle collecting unit.The second zone 11 is made of an electrically insulating plasticmaterial and the third zone 12 is made from a plastic, a conductingpolymer, for example having a PCT (positive temperature coefficient)effect upon the application of a voltage. This means that the electricalresistance of the plastic increases with increasing temperature. Suchplastics are known in the state of the art. The advantage with this typeof material is that it can be chosen so that a suitable heating effectcan be achieved whereby with an increase of temperature the heating ofthe polymer material reduces and finally stops completely. Thisbehaviour of the plastic is naturally reversible. This means that theconstruction of the heating part is simplified because no complicatedcontrol equipment is needed. In zone 12 two electrical conductors 15, 16are arranged, for example parallel with the longitudinal direction ofthe element and which are connectible to an electrical voltage sourcewith example 220 or 110 V alternating current. By connecting a voltage aresistive heating up of the material occurs.

These extended elements 20, 20' can be made through joint extrusion ofthe different materials whereby they form a continuous unit.

The second zone 11 can also be shaped with stamped out openings in orderto permit, for example, one or more corona wires to pass through theelements. The reason for this is described below.

FIG. 8 shows schematically a first embodiment of the supply air terminalcomprising an electrostatic filter according to the invention where theextended element according to

FIG. 7 is arranged essentially in the same way as described for theelectrostatic filter above, but where the regions of the elements whichare made of the electrically heatable plastic are arranged in connectionto and after the particle collecting unit.

The outer casing of the outlet is shown by 4. In the direction of theflow of the air (shown with arrows) are first arranged one or morecorona wires 5 with electrically insulating supports 6. Next is thefront edge of the element 20', which belongs to the group ofelectrically insulated elements, partially hidden by an element 20 lyingin front of it which belongs to the group of earthed elements. These twotypes are preferably arranged alternatively. The part of the elementswhich are referenced with 10 concerns the zone which has the specialmass resistivity which was mentioned earlier for the polymer materialsaccording to the invention, i.e. this part of the element canadvantageously be made from one or more non-hygroscopic polymermaterials with a mass resistivity which preferably lies within theregion 1×102 Ωcm to 1×10⁸ Ωcm, more preferably 1×10³ Ωcm to 1×10⁷ Ωcm,more precisely 1×10³ Ωcm to 1×10⁴ Ωcm. Zone 11 with the non-conductingplastic follows in the direction of flow of the air and then zone 12with electrical heating by means of conductors 15 and 16 which areconnectible according to the above.

FIG. 9 shows schematically a second embodiment of a supply air terminalcomprising the electrostatic filter according to the invention, wherethe elements according to FIG. 7 are arranged essentially in the sameway as described for the electrostatic filter above but where theregions of the elements which are made of the electrically heatableplastic are arranged in direct connection to and in front of theparticle collecting unit.

The outer casing of the outlet is shown by 4. First zone 12 withelectrical heating by means of conductors 15 and 16, which areconnectible according to the above, is arranged in the direction of flowof the air after which follows zone 11 on the elements whereby one ormore corona wires 5 are arranged in such a way that they can passthrough the openings 17 arranged in zone 11. This is the part of theelements which is made of the electrically insulating plastic. In thedirection of flow of the air there then follows the particle collectorzone wherein the front element 20' belongs to the group of electricallyinsulated elements and behind this there is a partially hidden element20 which belongs to the group of earthed elements. These two types ofelement are preferably arranged alternatively. The part of the elementswhich have the reference sign 10 concern the zone which has the specialmass resistivity which was given above for the polymer materialaccording to the invention, i.e. the part of the elements that can beadvantageously manufactured of one or more non-hygroscopic polymermaterials with a mass resistivity which preferably lies within theregion of 1×10² Ωcm to 1×10⁸ Ωcm, more preferably 1×10³ Ωcm to 1×10⁷Ωcm, more precisely 1×10³ Ωcm to 1×10⁴ Ωcm.

The embodiment according to FIGS. 7 to 9 can also be arranged in theshape of cylindrical elements in the same manner as shown in theembodiment according to FIG. 5 and FIG. 6.

Within the scope of the invention as defined in the claims it is alsoconceivable that extra elements are placed between those mentionedearlier in order to for example achieve a changed geometry, which in itsterm can give advantageous changes of the field distribution in thearrangement, for example one group of elements can be doubled or anextra element of the one or the other group can be replaced for similarreasons.

The invention is in no way limited to the above described embodimentsbut can be freely varied within the scope of the appended claims.

What is claimed is:
 1. In an electrostatic filter device for separatingparticles which comprises an upstream particle charging unit having atleast one corona wire for charging particles, and a particle collectorunit arranged relatively downstream of said particle charging unit; theimprovement wherein said particle collector unit comprises two groups ofplate elements arranged at a predetermined distance from each other andparallel to each other and parallel to the direction of flow through thedevice, a first group of plate elements being connected to earth, and asecond group of plate elements being electrically insulated withreference to the device and the first group of plate elements; and thecomposite constituent material of said plate elements being homogeneousand consisting essentially of one or more non-hygroscopic polymermaterials with a mass resistivity within the range of 1×10² Ωcm to 1×10⁸Ωcm.
 2. The electrostatic filter device according to claim 1, whereinthe mass resistivity ranges from 1×10³ Ωcm to 1×10⁷ Ωcm.
 3. Theelectrostatic filter device according to claim 2, wherein the massresistivity ranges from 1×10³ Ωcm to 1×10⁴ Ωcm.
 4. The electrostaticfilter device according to claim 1, wherein the elements are alternatelyoffset between themselves in the longitudinal direction of the elements,whereby the first group of elements are arranged furthest away from theparticle charging unit and are connected to earth, and the second groupof elements are electrically insulated with respect to the device and tothe first group of elements and arranged nearer to the particle chargingunit.
 5. The electrostatic filter device according to claim 1, furthercomprising holding-together means of essentially insulating material,arranged at a predetermined relative distance from each other forholding the elements.
 6. The electrostatic filter device according toclaim 5, wherein the holding-together means have the shape of a comb. 7.The electrostatic filter device according to claim 1, wherein thematerial in the elements is a plastic material doped with carbon.
 8. Theelectrostatic filter device according to claim 1, wherein the surface ofthe elements has a non-smooth uneven alternatively roughened surface. 9.The electrostatic filter device according to claim 1, wherein theelements have at least two zones with different resistivities separatedfrom each other by an area with a higher or lower resistivity.
 10. Asupply air terminal, which comprises as an integrated unit anelectrostatic filter device according to claim 1, and wherein at least apredetermined number of the elements comprised in the filter device areshaped so as to form a first zone in an enlarged element whichcontinuously at one end extends into a second zone with electricallyinsulating plastic, and the second zone continuously extends into athird zone which comprises a conducting plastic suitable for resistanceheating; said third zone has at least two conductors connectable to avoltage source, arranged for electrical resistant heating of at least apart of the third zone.